Such an apparatus is known from U.S. Pat. No. 4,785,176.
Such apparatus are known inter alia as Scanning Electron Microscope (SEM), Transmission Electron Microscope (MA) and Dual Beam Microscope (in which both an ion beam and an electron beam are employed). Apparatus as described above are used nowadays inter alia in areas of development, inspection and production of for example, the semiconductor industry. In this context, both, production means—such as lithographic. masks—and products and intermediate products—such as wafers—are inspected, repaired or manufactured in various phases using an electron beam and/or an ion beam. In irradiating a sample with charged particles, information can be obtained in various manners, such as with the aid of secondary particles and radiation excited in the sample. By collecting and processing this information with the aid of detectors, one can obtain insight into certain physical properties of the sample.
So as to prevent undesired electrical charging of, and/or damage to, the sample to be inspected, a certain landing energy will be required of the particles in the irradiating beam, dependent upon the nature of the sample. In the case of inspection using electrons, the desired landing energy typically lies in the range 0.5 to 5 keV.
Because of the constitution of the sample can vary from location to location, it may be desirable to adjust the landing energy commensurate with this varying constitution. In that context, it is desirable that the focus position of the beam does not shift with respect to the focusing device. This is of particular importance in a production environment, such as during the inspection of wafers, where adjustment of the focusing would disadvantageously influence the production throughput.
The magnetic lens in such apparatus is usually embodied with a coil that, with the aid of magnetic pole pieces, generates the focusing magnetic field. The current necessary for the generation of the magnetic field will cause heat dissipation in the coil. The physical dimensioning of the magnetic lens is determined to a great extent by the size of the coil and the space required for any cooling means that might be present. These cooling means, such as a cooling-water spiral, may be required so as to limit undesirable consequences of the dissipation, such as mechanical changes arising from temperature changes in the pole pieces.
In the field of particle-optical apparatus, there is a desire to miniaturize focusing devices as described here, for example so as to create space for detectors, or, with the aid of multiple beams of charged particles with attendant focusing devices, to concurrently obtain information from multiple locations on one or more samples.
In the US patent text referred to, a focusing device is described that consists of a magnetic lens for generating a focusing magnetic field with the aid of a coil and magnetic pole pieces, and an electrostatic lens for generating a focusing electric field, which electric field changes the energy of the beam. This known focusing device—by employing an electrostatic field that coincides wholly of partially with the magnetic field —strives to achieve a situation whereby the inevitable lens aberrations are smaller than the lens aberrations that can be achieved when using a magnetic lens alone. As regards the change in the energy of the beam, the US patent text referred to only states that the energy of the beam is decreased in the electrostatic field.